Acting in several appeals to the Privy Council for defendants who had been sentenced to death in Caribbean countries, his legal submissions led to the abolition of the mandatory death penalty in those countries. He worked with lawyers in African countries towards the same end. In 2005 he persuaded the House of Lords that evidence obtained by torture should be inadmissible in court. In 2007 he represented two alleged terrorists in a case in the House of Lords in which he successfully challenged their control orders on human rights grounds. He has also acted in 15 other cases in the House of Lords since 1999, including two cases about the conduct of British soldiers in Iraq, and representing David Shayler in his appeal against conviction for breaching the Official Secrets Act 1989. He gave free legal advice to the defendants in the "McLibel" case,[5] and was interviewed twice — ten years apart — in Franny Armstrong's 2005 documentary, McLibel.

On 22 July 2010 Starmer announced the controversial decision not to prosecute the police officer Simon Harwood in relation to the death of Ian Tomlinson resulting in accusations by Tomlinson's family of a police cover up.[7]

In the summer of 2012 British newspapers reported allegations that Starmer was personally responsible for the continued prosecution of Paul Chambers, a traveller who, frustrated at airport delays, had posted a joke about the airport on Twitter. In the case known as the "Twitter Joke Trial" Chambers had been convicted of sending a "public electronic message that was grossly offensive or of an indecent, obscene or menacing character". The trial and conviction provoked widespread protest by free-speech activists, but the Crown Prosecution Service maintained a long-term opposition to Chambers' appeals. According to Chambers' defenders, prosecutors had been willing to stop opposing the appeals, but Starmer had overruled his subordinates because he was "trying to save face by refusing to admit he was in the wrong."[12]

In December 2013 the Labour Party announced that Starmer would lead an enquiry into changing the law to give further protection to victims in cases of rape and child abuse. This raised speculation that he may be considering a political career with Labour.[15] On 28 December, Starmer refused to deny the possibility of entering politics, saying to BBC News "well, I’m back in private practice; I’m rather enjoying having some free time, and I’m considering a number of options."[16]